The present invention relates to a multi-colored organic electroluminescence (hereinafter referred to as xe2x80x9cELxe2x80x9d) device used for an organic EL display and the like.
A layered-type EL device, which exhibits a high luminosity of 1,000 cd/m2 or more under the applied voltage of 10V, was reported by Tang et al. in Appl. Phys. Lett. vol. 51, p. 913 (1987). An organic EL device has been actively researched to aim at practical use since the report. The organic EL device is a thin film device of spontaneous emission of light, and has features of low driving voltage, high resolution, and wide-angle visibility, which are not obtainable by other elements. The organic EL device is expected to apply to a flat panel display, and for wide applications of the organic EL element, it is indispensable to be displayed by multi-color.
A first method for the multi-colored display is to utilize organic EL elements having the three primary colors, wherein a plurality of pixels for the three primary colors is arranged by successively patterning on the same plane. A second method for the multi-colored display is to utilize the EL cells emitting white light and three kinds of color filters disposed opposite to the EL cells, each of the color filter transmitting red, green or blue light.
However, patterning of the EL elements for the three primary colors decreases the luminescence efficiency of the elements.
Moreover, the complicated processes of the patterning of the pixels for the three primary colors render the mass production of the multi-colored organic EL device difficult. Further, an electro-luminescent material for red color, in particular, with good purity has not been found. Thus, the first method for the multi-colored organic EL device has not bet been used practically. The second method utilizing the color filter has not been used practically either, since there is no organic EL cell with stable and ample luminosity in white light.
Accordingly, a third method utilizing a color conversion filter has been developed in recent years (Japanese Unexamined Patent Publications No. 3-152897 and No. 5-258860).
The color conversion filter comprises a material which absorbs luminescent light emitted by an organic EL cell and emits fluorescent light with the wavelengths in the visible region. Because the luminescence of the organic EL cell in the color conversion method is not limited to white light, it is possible to utilize an organic EL cell with higher luminosity at an appropriate wavelength other than white light. In fact, a color conversion method performs at a conversion efficiency of 60% or more by utilizing a blue light emitting organic EL cell and a color conversion filter which converts the blue light to light with a longer wave length.
Manufacturing an organic EL device in the color conversion method requires special consideration on the distance between the color conversion filter and the organic EL cell. As the distance becomes longer, the light leakage from the adjoining pixel increases, so that angle of visibility becomes poor. Consequently, a preferable configuration is a direct mounting of the organic EL cell on the color conversion filter. Known phosphor pigments for the color conversion filter are rhodamine pigments, pyridine pigments, oxazine pigments, and coumalin pigments, etc. (Japanese Unexamined Patent Publications No. 8-78158, No. 8-222369, No. 8-279394, No. 8-286033, No. 9-106888, No. 9-208944, No. 9-245511, No. 9-330793, and No. 10-12379). However, the conventional phosphor pigments are often susceptible to the factors, such as ultraviolet radiation, heat or organic solvents, and result in shifting of fluorescent wavelength or quenching of fluorescence. Therefore, when the organic EL cell is directly formed on the color conversion filter, the function of the color conversion filter may disappear due to plasma in a sputtering process for a transparent electrode or peeling liquid used in a patterning process of the transparent electrode.
The thicknesses of the color conversion filters corresponding to the respective three primary colors are different from each other because differences in the color conversion efficiencies of fluorescent materials for the three primary colors are adjusted by the film thicknesses of the color conversion filters to yield desired tone of color. Accordingly, steps are formed in the color conversion filters 2, 3, 4 on a glass substrate 1, as shown in FIG. 4. Direct formation of the organic EL cells on the uneven color conversion filters often causes disconnection of the transparent electrodes and irregular film thicknesses of organic light-emitting layers, rendering the luminescence from the organic EL cells unstable.
A conventional liquid crystal display is provided with a protective layer on the color filters. The protective layer insulates the substrate and makes the substrate flat, and prevents physical destruction of the elements of the display. The materials studied so far for the protective layer include acrylic resin (Japanese Unexamined Patent Publication No. 60-216307), epoxy resin (Japanese Unexamined Patent Publications No. 4-97102, and No. 3-8652), and polyimide resin (Japanese Unexamined Patent Publication No. 1-229203).
However, the conventional materials mentioned above require ultraviolet irradiation or heat-treatment at high temperature of 200xc2x0 C. or more to form the protective layer. These processes deteriorate the characteristic of the color conversion filter underneath the protective layer. Hence, the conventional materials are not suitable for use as a protective layer of a color conversion filter.
Accordingly, it is desirable to develop a new material for the protective layer, which enables direct formation of the organic EL cells on the color conversion filters. The protective layer is required to be formed without deteriorating performances of the phosphor pigments in the color conversion filter, to provide flat surface absorbing the steps of the color conversion filters, and to protect the phosphor pigments in the subsequent processes.
The present invention has been made in view of the above problems. An object of the present invention is to provide a multi-colored organic EL device having a protective layer for color conversion filters allowing organic EL cells directly formed on the protective layer.
Another object of the invention is to provide a multi-colored organic EL device as stated above, wherein the protective layer can be coated on the color conversion filters without deteriorating the phosphor pigments in the color conversion filter, and to flatten the uneven color conversion filters.
A further object of the invention is to provide a multi-colored organic EL device as stated above, wherein the protective layer can protect the phosphor pigments from the post-processing.
A still further object of the invention is to provide a method for manufacturing a multi-colored organic EL device.
Further objects and advantages of the invention will be apparent from the following description of the invention.
Intensive investigations lead the inventors to achieve the above described objects. The protective layer for the color conversion filter in the invention utilizes a coating resin which enables forming a protective film at a temperature at which the function of the color conversion material does not disappear. The protective layer has a glass transition temperature (hereinafter referred to as xe2x80x9cTgxe2x80x9d) of higher than 100xc2x0 C., and has a hardness of 2H on the pencil hardness. The protective layer can be formed without deteriorating performances of the color conversion filters and effectively protects the color conversion filters against stresses during depositing layers of the organic EL cell on the protective layer.
The multi-colored organic EL device of the invention comprises a transparent substrate, a plurality of different phosphor layers arranged separately in a plane on the surface of the substrate, an organic EL cell formed above the phosphor layers, and a protective layer between the phosphor layers and the organic EL cell. The EL cell comprises an organic light-emitting layer that emits light when electric charges are injected to the light-emitting layer. The phosphor layers and the light-emitting layer are arranged in such a manner that luminescence from the light-emitting layer irradiates the phosphor layers to cause the phosphor layers fluorescent. The coating resin of the protective layer has a curing temperature at which quenching of fluorescence in the phosphor layers does not occur. The glass transition temperature of the protective layer is more than 100xc2x0 C., and the surface hardness is more than 2H on the pencil hardness.
One of the phosphor layers can be replaced by a color filter to attain light with the same color purity as the light emitted from the organic light-emitting layer of the organic EL cell.
The coating resin preferably includes one of polynorbornene resins with number average molecular weight from 50,000 to 500,000. Alternatively, the coating resin may include a polymer hybrid resin containing alkoxysilane selected from trialkoxysilane and tetraalkoxysilane.
Moreover, the method for manufacturing a multi-colored organic EL device comprises a step of forming an organic EL cell directly on the protective layer.
Application of the coating resin of this invention to the color conversion filters enables flat-coating for the uneven color conversion filters with the protective layer of film thickness of 10 xcexcm or less. The organic EL cell is formed directly on the protective layer. This makes the organic EL cell very close to the color filter to provide the multi-colored organic EL device exhibiting a characteristic of wide-angle visibility. The process for preparing a protection structure for the color conversion filter is very simple because the process is only to coat at least one layer on the color conversion filter.